Add Date.now() to MockDate.
Revert "Add Date.now() to MockDate."

This reverts commit a8fe9e434d77abfaa728f28d99f8eedf12eae5f1.
Add Date.now() to MockDate (without updating timecop-0.1.1.js this time).
Merge pull request #22 from jwark/add_Date_now

Add Date.now() to MockDate.